If a mobile station moves from a specific mobile communication region to another mobile communication region, handover which automatically converts a communication channel to prevent communication from being disconnected is performed. The general procedure of performing such handover will be described as follows. FIG. 1 is a flow chart illustrating the procedure between a mobile station and base stations to sequentially illustrate the general procedure of executing handover according to the related art.
A service base station transmits handover triggering information to the mobile station through a downlink channel descriptor (DCD) for transmission of channel information or message (MOB_NBR_ADV) for transmission of information of neighboring base stations (S101), wherein the handover triggering information designates trigger conditions of various operations of the mobile station with respect to handover, i.e., handover related operations. One example of lists included in the handover triggering information is as follows.
TABLE 1LengthNameType(1 byte)ValueType/54.11See Table 358b for descriptionFunction/ActionTrigger54.21Value to be compared with measurementvaluemetric value to decide trigger conditionsTrigger54.31Averaging duration of metric measurementaveragingvalue in mobile stationduration
Also, the handover triggering information may further include the following lists.
TABLE 2NameLengthValueType2 bits0x00: CINR metric(MSB)0x01: RSSI metric0x02: RTD metric0x03: ReservedFunction3 bits0x00: Reserved0x01: metric of neighboring base station is greater than absolute value0x02: metric of neighboring base station is smaller than absolute value0x03: metric of neighboring base station is relatively greater than metric of service base station0x04: metric of neighboring base station is relatively smaller than metric of service base station0x05: metric of service base station is greater than absolute value0x06: metric of service base station is smaller than absolute value0x07: ReservedEnabled3 bits0x00: ReservedAction(LSB)0x01: MOB_SCN-REP message response after each scanning interval to trigger0x02: MOB_MSHO-REQ message response to trigger0x03: mobile station initiates scanning of neighboring base station by transmitting MOB_SCN-REQ messagein response to trigger0x04: mobile station is advised to perform scanning during trigger0x05~0x07: Reserved
Afterwards, if the mobile station triggers scanning initiation of a neighboring base station to a service base station as a predetermined metric received from the neighboring base station satisfies scanning initiation condition (0x03 of Action list) of the handover triggering information (S102), the mobile station transmits a scanning request message (MOB_SCN-REQ) to the service base station, and the service base station allocates a predetermined scan duration and at the same time transmits a scanning response message (MOB_SCN-RSP) to the corresponding mobile station in response to the scanning request message (MOB_SCN-REQ). The mobile station synchronizes a downlink signal with the neighboring base station for the allocated scan duration and measures quality level of the signal received from the neighboring base station, strength of the signal, transmission synchronization, etc. to perform scanning of the neighboring base station (S103).
If the mobile station triggers scan reporting to the service base station as the scanning result satisfies scan report condition (0x01 of Action list) of the handover triggering information (S104), the mobile station transmits carrier to interference and noise ratio (CINR), received signal strength indication (RSSI) and round trip delay (RTD), etc., which are measured through the scanning, to the service base station through the MOB_SCN-REP message.
If the mobile station triggers handover request to the service base station as the scanning result satisfies handover condition (0x02 of Action list) of the handover triggering information (S105), the mobile station transmits the handover request message (MOB_MSHO-REQ) to the service base station, and the service base station transmits the handover response message (MOB_MSHO-RSP) to the corresponding mobile station in response to the handover request message. Subsequently, the mobile station transmits a handover indication message (MOB_HO-IND) to the service base station to request release of connection with the service base station (S106) and then performs the network re-registration procedure with a handover target base station.
Meanwhile, a broadband wireless access system based on IEEE802.16e system supports a power saving mode for minimizing power consumption of the mobile station. The operation of the mobile station in the power saving mode is performed by repetition of a sleep interval and a listening interval. The sleep interval and the listening interval are respectively determined by each value of a sleep window and a listening window. In this case, the sleep interval and the listening interval may have different values depending on characteristics of traffic set in a corresponding mobile station. The power saving mode can be classified into power saving mode class of type 1, power saving mode class of type 2, and power saving mode class of type 3.
The mobile station which is being operated in the power saving mode can perform scanning of the neighboring base station even though the mobile station does not receive and transmit the scanning request message (MOB_SCN-REQ) and the scanning response message (MOB_SCN-RSP) from and to the service base station. If the mobile station transmits the scanning report message to the service base station whenever performing scanning of the neighboring base station, a problem occurs in that power consumption of the mobile station is inevitably required unlike intention of the power saving mode. However, if the base station does not receive the scanning report message for handover from the mobile station, a problem occurs in that the base station cannot control handover to the mobile station of the power saving mode appropriately.
Furthermore, a problem occurs in that the current mobile station fails to effectively perform handover related operation and power saving operation as the mobile station does not use characteristics of each class by performing handover related operation regardless of the power saving class which is being operated.